David Cameron today sought to position the Conservatives as a serious alternative to Gordon Brown's government as he told the prime minister: "You have had your boom and now your reputation is bust."

Addressing delegates at the Tory conference in Birmingham, the Conservative leader said his party was united and had a clear plan to deal with the economic turmoil.

Earlier, Cameron admitted the Conservatives had not "sealed the deal" with voters, but he added: "You never seal the deal until you really win people's trust ... a battle we have to fight every day, every week, every month."

Cameron cancelled the showing of a video promoting the party's achievements this year to reflect the anxious mood of the country during the credit crunch.

A BPIX survey for the Sunday Telegraph today put the Tories on 43%, down four, Labour on 31%, up seven, and the Liberal Democrats up one at 17%. But the same poll showed that Gordon Brown and Alistair Darling were by one percentage point regarded as better able to deal with the economic crisis than Cameron and George Osborne, the shadow chancellor. The Tories are conscious that they need to use this week to demonstrate that they are ready for the hard realities of government.

Setting out the party's reconstruction plan to deal with the economic turmoil, Cameron told the conference that a Conservative government would create an independent body to monitor state spending and would provide a beefed-up role for the Bank of England in overseeing the financial sector.

Insisting his party was committed to helping people cope with the rising cost of living and the impact of the credit crunch, the Tory chief said people were crying out for change.

"Let us show them that this is the end for the big spending, big taxing, recklessly borrowing, big, bossy, interfering government that promises so much and delivers so little," he said. "We have a plan for change, so let's show them that we can be that change."

Under a package of measures unveiled by Osborne today to prevent future bank collapses, the Bank of England would take a broader responsibility for debt. The bank would write a write a regular, open letter to the Financial Services Authority setting out its assessment of the extent of risk in the market.

The Tories would also offer savers deposit insurance worth £50,000, to be funded by the financial services sector and paid out within a week of a bank collapsing, more than the current level of £35,000 offered by the government.

A new, independent office of budget responsibility would audit the nation's debts, including liabilities currently not counted on the balance sheet by Labour, such as major PFI projects.

Contrasting his party with the disunity at the top of the Labour government, Cameron told conference: "I want us to show that, at a time when the government has completely lost its way, that there is a very clear, a very strong, a very united and a very positive alternative: the modern Conservative party."

Labour, he said, had spent their conference last week "talking about themselves to themselves".

The Tories would be able to "spend all of our time talking about the problems people face in our country and the changes we want to make as a party and, if elected, as a government."

It was important for the party to avoid becoming complacent because "we have to win so many people's trust about how we will be able to take this country forward", Cameron said.

"We are meeting at a time of great economic difficulty and a time of huge anxiety ...

"My message to Gordon Brown is this: 'You have had your boom and your reputation is now bust.'"

Cameron topped the billing at the opening of the four-day event in the Midlands. Tory members were also treated to an address by Boris Johnson, the mayor of London, who succeeded in injecting a note of seriousness into an otherwise hilarious speech.

Between jokes at the expense of the "monosyllablic Austrian cyborg" Arnold Schwarzenegger, the governor of California, Johnson saidLondoners would not face increased council tax bills to pay for the 2012 Olympics.

William Hague, the shadow foreign secretary, took to the stage to attack government incompetence, while Osborne condemned Brown's economic record

Caroline Spelman, the party chairman, stressed the need to stick to the centre ground and stay united in the face of Labour "infighting and introspection".